The invention relates to a paste for welding ceramic materials to metallic contact surfaces or metals according to the species defined in the principal claim, as well as a method for producing a welded join between ceramic materials and metals using this paste.
It is commonly known that metallic connecting contacts can be implemented on ceramic substrates by way of bonding, welding, or soldering methods. The contacts and contacting methods known so far are not suitable for high mechanical and thermal stresses, however, since the adhesion between the connecting contact and ceramic substrate is often insufficient. Plug contacts have hitherto been used in such cases.
The object of the present invention is therefore to develop a paste, and a method that can be carried out therewith, for welding metallic contact surfaces or metals to ceramic materials, in which context the paste withstands high thermal and mechanical stresses so as thereby to avoid the use of cost-intensive standard plug contacts.
The paste according to the present invention having the characterizing features of the principal claim, and the method carried out therewith for welding metallic contact surfaces to ceramic materials, has the advantage, as compared to the existing art, of making possible direct welding of metallic contact surfaces, for example lead frame materials, to ceramic standard materials via an intermediate layer made of the paste; the weld, while having good electrical properties, at the same time withstands high thermal and mechanical stresses, so that plug contacts hitherto used can be omitted. A considerable reduction in the cost of manufacture is thus also achieved.
The paste according to the present invention thus makes possible in simple fashion, for example, electrical contacting to sensor elements using welding methods, in which context metallic materials are welded onto standard substrates and the paste according to the present invention acts as an adhesion layer. The temperature resistance of the welded join is guaranteed up to at least 500xc2x0 C.
Advantageous developments of the invention are evident from the features recited in the dependent claims.
As a result of further added paste components such as a binder, a softener, a solvent, a dispersing agent, and a thixotropic agent, it is possible in particular to influence the processability of the paste according to the present invention very advantageously in terms of viscosity and handling, so that it is also suitable, for example, as a printing paste or for application by way of a ductor, and can thus easily be integrated in terms of process engineering into existing production techniques.
It is also advantageous if, prior to the sintering of the ceramic material and paste, a platinum conductive paste is additionally printed onto the paste that has been applied onto the ceramic material and, in particular, dried. The electrical conductivity of the welded join can thereby be improved, in particular if the paste according to the present invention contains low concentrations of noble metal.
The paste according to the present invention contains at least one noble metal, at least one aluminum compound, at least one silicon compound, and at least one compound selected from the group of the barium compounds, magnesium compounds, and calcium compounds.
Moreover, in an advantageous development of the invention, a binder, a softener, and a solvent are additionally added for simplified handling and production of the paste. In addition, if necessary, a thixotropic agent and a dispersing agent are incorporated into the paste if necessary. Platinum, palladium, silver, or gold is especially suitable as the noble metal.
Suitable barium compounds are, in particular, barium oxide, barium carbonate, barium hydrate, barium acetate, or another organic barium compound; it is very advantageous if the barium compound, during subsequent drying and/or sintering of the paste according to the present invention together with the ceramic material and the metallic contact surface, decomposes or is converted into barium oxide as completely and with as few residues as possible. Also particularly advantageous is the addition of barium oxide as the barium compound to the paste according to the present invention, since, for example, vigorous gas evolution or the release of water is disadvantageous in the context of welding.
The barium compound in the paste according to the present invention can be entirely or partially replaced by a magnesium compound and/or a calcium compound; MgO, CaO, MgCO3, CaCO3, the hydroxides of calcium or magnesium, and organic magnesium or calcium compounds are particularly suitable for this purpose, since after drying and/or sintering they very advantageously, similarly to the requirements for the barium compound, are present at least largely as corresponding metal oxides.
Silicon dioxide, an organic silicon compound, or a silicon-oxygen compound is suitable as the silicon compound; it is again very advantageous if the silicon compound, during subsequent drying or sintering of the paste according to the present invention together with the ceramic material and the metallic contact surface, decomposes or is converted into silicon dioxide as completely and with as few residues as possible. The addition of silicon dioxide, kaolin, kaolinite, or Mg2(SiO4) (forsterite) as the silicon compound is particularly preferred.
Aluminum oxide (Al2O3), an organic aluminum compound such as aluminum formate, or aluminum hydroxide (Al(OH)3 or AlO(OH)) is suitable as the aluminum compound; here again, as in the case of silicon and barium, it is very advantageous if the aluminum compound, during subsequent drying or sintering of the paste according to the present invention together with the ceramic material and the metallic contact surface, decomposes or is converted into aluminum oxide (Al2O3) as completely and with as few residues as possible. Direct addition of aluminum oxide (Al2O3) as the aluminum compound is particularly preferred.
A paste using platinum, silicon dioxide, aluminum dioxide, and barium oxide has proven to be very particularly preferred in terms of adhesion, economy, and simple handling.